A method is known for operating a diesel engine that comprises four cylinders and a piezo injector for each cylinder for injecting fuel into each of the four cylinders. A piezo injector is an injector in which the valve tappet is operated by a piezo actuator. During the known method, the torque of the diesel engine is first detected during an overrun phase of said engine. Then, the injectors are actuated with varying actuation times τinj and then the torque variation is used to determine how much fuel has been injected by each piezo injector.
In this way, the actuation time that is required for injecting a specified injection quantity into the combustion chamber is determined and, if necessary, an actuation time correction is determined.
Based on the actuation time correction determined in this way, the actuation time τinj for all injectors is altered such that the quantity of fuel specified by an engine management system is always injected. This method is only used for small injection quantities, as otherwise the running smoothness of the internal combustion engine suffers in the overrun phase and noises occur. This method reduces fuel consumption and the emission of pollutants.
The drawback of the known method is that this method makes high demands on the production accuracy of the piezo actuators. That is to say, if the piezo actuator exhibits too much elongation at a given actuation voltage, a shortest possible actuation time specified by the engine management system can result in too great an injection quantity. Therefore, narrow tolerances must be complied with for piezo actuators. In addition, ageing of the piezo actuators can also cause the quantity of fuel that is injected during the shortest possible actuation time to rise. Then, a small injection amount to be injected that is specified by the engine management system may be exceeded.